Field of the Invention
The present invention relates to a polishing apparatus for polishing a substrate, such as a wafer, and more particularly to a polishing apparatus having a function to diagnose a condition of a polishing pad or a dresser to determine the end of its life.
Description of the Related Art
In fabrication of semiconductor devices, planarizing a surface of a substrate is a very important process. A typically used technique for the surface planarization is chemical mechanical polishing (CMP). In this chemical mechanical polishing, the surface of the substrate is polished by bringing the substrate into sliding contact with a polishing surface of a polishing pad, while supplying a polishing liquid that contains abrasive grains, e.g., silica (SiO2), onto the polishing surface.
This chemical mechanical polishing is conducted using a CMP apparatus. This CMP apparatus has a polishing table for supporting the polishing pad and a top ring for holding the substrate. The polishing pad is attached to an upper surface of the polishing table. During polishing of the substrate, the polishing table and the top ring are rotated about their own axes, and the top ring presses the substrate against the polishing surface of the polishing pad to thereby provide sliding contact between the substrate and the polishing pad. The polishing liquid is supplied onto the polishing surface of the polishing pad, so that the substrate is polished in the presence of the polishing liquid between the substrate and the polishing pad. The substrate surface is planarized by a combination of a chemical polishing action of alkali and a mechanical polishing action of the abrasive grains.
As polishing of the substrate is performed, the abrasive grains and polishing debris adhere to the polishing surface of the polishing pad, lowering a polishing performance thereof. Thus, in order to regenerate the polishing surface of the polishing pad, pad dressing is performed by a dresser. This dresser has hard abrasive grains, such as diamond particles, fixed to a lower surface thereof and is configured to scrape away the polishing surface of the polishing pad to thereby regenerate the polishing surface.
The polishing pad is worn down gradually by the pad dressing. Since the worn polishing pad cannot exhibit its intended polishing performance, it is necessary to replace the polishing pad regularly. Conventionally, the replacement of the polishing pad is typically determined based on the number of substrates polished. However, the number of substrates polished does not necessarily reflect an accurate end point of the life of the polishing pad. For this reason, it is necessary to replace the polishing pad before the end of its service life in order to maintain the polishing performance. Moreover, such frequent replacement of the polishing pad would result in a low operating rate of the CMP apparatus.
For the purpose of avoiding such frequent replacement of the polishing pad, it has been developed a polishing apparatus configured to measure a surface position of the polishing pad (i.e., a pad height) and to monitor wear of the polishing pad based on the measured values (for example, see Japanese laid-open patent publication No. 2002-355748). This type of polishing apparatus can determine the end of the life of the polishing pad based on the measured surface position of the polishing pad, i.e., an amount of wear of the polishing pad.
However, individual polishing pads may have different thicknesses and may have grooves with different depths formed on the surface thereof. As a result, it has been difficult to accurately determine the end point of the life of the polishing pad from the surface position of the polishing pad.
The abrasive grains of the dressers are also worn down gradually by pad dressing. The lowered dressing performance can lower the polishing performance of the polishing pad. Therefore, it is necessary to replace the dresser regularly, as well as the polishing pad. The polishing pad and the dresser are consumables of the polishing apparatus, as described above, and there has recently been an increasing need to minimize costs of these consumables. In order to achieve the cost reduction of the consumables, it is necessary to accurately determine the replacement times of the polishing pad and the dresser, i.e., the lives of these consumables.